Well, its form could conceivably be different. "Generally accepted medium of exchange" is one definition. But in the case of national fiat, it's also the unit o

Well, its form could conceivably be different. "Generally accepted medium of exchange" is one definition. But in the case of national fiat, it's also the unit of account and store of value. More broadly, it's an accounting device for intertemporal coordination of savings-investment decisions. Private currencies would be especially apt in the latter case. Certainly some systemic cooperation would be in order to maintain reflux, but it's more reliable than deposit insurance when people's equity is at stake. Social safety nets can be ensured from a myriad of techniques.